Is it worth it to buy the paid version of an app if you can download another version for free? If you value your battery life, it very well could be.

A new study conducted by researchers at Purdue University found that 65% to 75% of the energy used by free apps is committed to user tracking, downloading ads and uploading user information. That means only 25% to %35 of the energy is used for performing tasks like, you know, actually running the app’s core function.

Computer scientist Abhinav Pathak and his team analyzed several Android apps including Angry Birds and the most popular free chess app in the Android Market. For free games, downloading ads over 3G was by far the biggest energy waster. In Angry Birds’ case, only 18% of the energy used was spent on actually running the game.

Apps designed by news organizations spend a lot more energy collecting information about you. According to the study, the New York Times’ app committed 20% to 30% of its energy use to tracking user data.

The reason why your battery dies so fast is that apps aren’t so efficient when it comes to uploading and downloading information over 3G, Wi-Fi and GPS connections, staying in long periods of high-energy use when they probably don’t need to — researchers fixed this problem by restructuring several apps to reduce their energy consumption by 20% to 65%.